The present invention deals with a method and an arrangement for producing synthetic plastic molded parts. In a so-called reaction-injection molding of synthetic plastic molded parts, two initially liquid reaction components, such as polyetherpolyol and polyisocyanate, are supplied by a dosing pump to a mixing head and united to form a reaction mixture. The reaction mixture flows from the mixing head via a sprue passage into a molding tool and reacts there to form a solid material, for example polyurethane. For providing swirling-free supply of the mixture, the flow from the cylindrical outlet passage of the mixing head is converted into a gap flow by a distributing device which includes a limiting member or beam. A film sprue is formed in the region of the sprue gap. The film sprue must have a certain width in correspondence with the respective molded part. In the event of a material with a filler, for example with a glass fiber filler, a certain gap thickness must be maintained, for ensuring optimal flow conditions for the mixture introduction. In the event of an asymetrical geometry of molded parts and the use of filled materials, it can be necessary to provide a material overflow on the side opposite to the sprue, for avoiding air inclusions. The overflow takes place also through a narrow gap similar to the sprue gap.
The sprue part which is formed on the hardened molded part must be removed from the molded part after opening of the mold outside of the molding tool, by means of special tools or manually by cutters or cutting templates. This separation must be performed accurately, since the contour of the cut simultaneously forms the outer contour of the molded part. Moreover, in this case the outer skin of the molded part is destroyed, which in the event of foam material parts with cellular structure results in the cells no longer being closed by the outer skin.
The German document DE-A-No. 1 924 022 proposes a solution of this problem, in accordance with which a blocking device is provided between the sprue passage and the mold nest. The blocking device includes a blocking piston which moves in an opening of the upper part of the molding tool and in the closed position abuts with its end surface against a lower part arranged on the separating wall. In the open position of the blocking piston, the sprue passage is connected with the mold nest via a released opening space. The reaction mixture follows this path from the mixing aggregate to the mold nest. After the end of the filling process, the blocking piston closes the mold nest, so that the sprue part located in the sprue passage is separated from the molded part, and after opening of the molding tools can be removed independently from the mold part. The blocking piston which is guided in the cylindrical opening extends at both sides over the sharp-edged separating wall. Thereby in the opening position a slot is formed, which is limited by the opening edge and located between the piston and the separating wall. The reaction mixture enclosed in this slot is sharply deflected, which leads to a swirling and to pressure differences responsible for material non-uniformity in the molded part because of the formation of air inclusions and pores in the molded part. Furthermore, the swirling causes a flow reorientation and even a reverse in the material stream. In the synthetic plastic materials with fillers, such as glass fibers, this leads to nonhomogeneity in the orientation of the fillers. Similar difficulties occur at the other end of the molding tool opposite to the sprue point or also inside the mold nest, when for example it is necessary to produce large-surface parts provided with hollows and made of filled materials.